The treatment of major tendon and ligament injuries is a significant therapeutic problem. In many cases, repair or reconstruction of the injured structure is necessary to reduce instability. These repairs may also serve to slow the progress of associated degenerative joint disease (arthritis). Although tendon and ligament reconstructions using grafts of the patient's own tissue (autografts) have been successful, they require additional surgery for graft preparation. This alone may result in significant problems at the harvest site. These grafts may also undergo elongation or mechanical failure. Tendon and ligament grafts obtained from human cadaveric donors (allografts) can produce an immune response and fear of viral contamination have further curtailed their use. In spite of these concerns, demand for allograft ligament substitutes far exceeds the supply of available tissue; thus an alternative is clearly needed. Synthetic grafts avoid the additional surgery of graft harvest. However, synthetic materials present a host of problems including attachment, wear and wear debris and lack of mechanical and biological compatibility. Graft materials from animals (xenografts) are readily available. The obvious disadvantage of this material is its significant rejection (immune) response. Extraction procedures have been developed to remove the antigenic components of bovine tendon xenografts and greatly reduce the tendency for rejection. In rabbits, the mechanical strength, biocompatibility and humoral immune response to processed bovine xenografts has undergone testing. Although the initial results have been extremely promising, before human use can be considered these xenograft procedures must be optimized and more extensively tested. If the proposed experiments are successful, the extraction procedures developed for xenografts may permit the use of these materials in human ligament surgery.